"I asked for my release," Bethea told ESPN's Josina Anderson on Tuesday. "I think it was a better fit for both parties. I want to take some time to explore my options. I still got some ball left in me."
Bethea, 32, played in all 16 games in two of the three seasons he spent with the Niners. He led the team with 110 total tackles last season and also had one interception.
"Antoine is a true professional in every sense and I have a great deal of respect for the way he has gone about his business throughout his career," general manager John Lynch said in a statement. "We would like to thank him for the positive impact he has had on this team both as a player and as a leader, on and off the field. We wish Antoine, and his family, great success in the future."
A three-time Pro Bowler, Bethea started each of the 162 regular-season games in his 11-year career that began in Indianapolis with the Colts.
With Bethea released, the 49ers gain $5.75 million against the cap to give the team nearly $100 million in cap space heading into Thursday's official start of free agency.
Information from ESPN's Nick Wagoner contributed to this report.